Designing and Constructing the Transmission Line

A variety of transmission line structure designs may be used for the project. The structures may be a combination of single and double circuit towers and steel lattice and tubular steel towers.

Idaho Power follows all applicable safety codes and standards to ensure our systems operate safely and reliably. The National Electric Safety Code (NESC) is the industry–accepted safety standard which guides the design, installation, operation and maintenance of transmission lines and equipment.

The approximate proposed design standards are listed below.

Tower heights

  • Steel lattice: 135 to 180 feet
  • Steel pole H-frame: 100 to 165 feet
  • Steel pole: 130 to 150 feet

Tower footprint: 40 by 40 feet

Span length: (average): 1,100 to 1,200 feet

Ground clearance: (average): 35 feet

Right of way width: 250 feet

If the federal and state agencies approve a transmission line route, Idaho Power will work with affected property owners to determine the exact line location and identify opportunities to reduce impacts to the property. Visit our People and Power Lines section for more information.

Image showing the potential family of transmission line structures that would be used to support the transmission lines. The image includes towers with heights ranging between: 140 and 190 feet for a single-circuit, steel lattice tower, 100 and 165 feet for a single-circuit, steel H-frame tower and 130 and 150 feet for a single-circuit steel pole. In addition, the image shows a 250 foot right of way for each tower structure.

Perspective photos
Example image showing the transmission line's visibility from a distance.

To view how a typical 500 kV transmission line looks from a distance of 500 feet to two miles away, visit our Perspectives Photos page in the Photo/Video Gallery.


Construction

Construction of the proposed transmission line would begin after Idaho Power receives permits from BLM and USFS, and an Energy Facility Site Certificate from ODOE-EFSC.

Constructing a transmission line is a big undertaking, which includes building access roads and towers, stringing lines and restoring disturbed lands. Idaho Power will oversee all transmission line and substation design and construction, and contract with construction companies to build the line.

Idaho Power will conform to all state and federal siting requirements when siting the line. Additionally, all applicable safety codes and standards will be followed to ensure our system operates safely and reliably.


The video illustrates the construction of a 500 kV line using galvanized steel structures.

Construction Sequence

The sequence for general transmission line construction is described below.

Image showing a helicopter lifting a transmission line structure and showing a construction site with large spools of cable. .
Building access roads and site preparation
  • Access roads are built throughout the corridor to construct the transmission line and conduct maintenance during the life of the line. In order to accommodate large equipment and materials during construction, access roads are approximately 14 feet wide.
  • Construction staging areas and substation sites are also constructed.
  • Erosion, both during and after construction, is minimized with the installation of water bars, culverts and sediment basins.
Installing tower foundations
  • Foundations are installed at each tower site. Typical foundations are made of steel reinforced concrete piers and are generally four feet in diameter and 15 feet deep, though these numbers may vary depending on the soil and/or rock type at each site. Each foundation extends about two feet above the ground.
  • Reinforced steel anchor bolt cages are installed after excavation and before concrete placement to strengthen the foundation’s structural integrity.
Assembling the structure
  • Components for each structure are delivered by flatbed trucks and assembled onsite by a truck-mounted crane. The structures are then lifted onto the foundation using a large crane specifically designed for tower construction. The crane moves along the right of way to set each structure in place. In some locations, it is necessary to set structures using a helicopter (see photo).
  • Every three miles along the right of way, temporary five-acre work sites are set up for the equipment used to pull and tighten the wires or conductor.
Stringing the line
  • The initial stringing begins with a helicopter pulling a lighter weight sock line through sheaths attached to each tower. A specialized wire-stringing vehicle is attached to the line to pull it through, followed by tightening or tensioning the line to achieve the correct sagging of the line between support structures.
Restoring and re-vegetating disturbed lands
  • Disturbed areas around the structures are restored and re-vegetated, as required by the land management agency and per the property owner agreement. All practical means are used to return land to its original contour and natural drainage patterns along the right of way.

This website is the joint Boardman to Hemingway Transmission Line Project site. Information contained on this site is approved for posting by Idaho Power. Visit the Idaho Power website.